Whenever you have time, can you please provide some more details related MIRV/S-400. As per my understanding, MIRV is to counter the ABM system.
Generally in order to successfully intercept ballistic missiles...it has to be done before reentry. After reentry the warhead picks up so much speed and if it is not purely ballistic(as in can manuever)...it practically becomes impossible to intercept then. In order to do this...it helps immensely to detect the missile launch as soon as it happens(within enemy territory or out on the sea in case of a boomer)...
...US has created THAAD for this very purpose(to intercept ballistic missiles). However it relies on a ton of other sensors and US makes everything for its defense needs so various different systems can have seamless integration. For example US has various satellites capable of detecting IR signature of things like ballistic missiles(among other things). This info can then be passed on to other sensors like radars for tracking said missile right away...to track/calculate trajectory, speed, etc. all of this culminating in THAAD trying to intercept it before reentry.
S400 on the other hand...is more suited to shooting down fighter jets, drones, cruise missiles, etc. Russia is developing S500...which is more like a THAAD analogue of sorts.